A semiconductor laser typically includes a substrate, a first confinement layer overlying the substrate, an active layer overlying the first confinement layer and a second confinement layer overlying the active layer. The refractive index of the active layer is greater than that of the first and second confinement layers thereby providing an optical guide which confines the light beam generated therein to operate in the fundamental mode in the direction transverse to the plane of the layers. A buried heterostructure laser provides an optical guide in the lateral direction, the direction in the plane of the layers and perpendicular to the direction of propagation of the light beam, by embedding a mesa which includes the active and confinement layers in a lower refractive index burying region. Ettenberg et al., in U.S. Pat. No. 4,048,627, has disclosed such a laser having a cladding layer interposed between the mesa and the burying region with a p-n junction therebetween to block current flow through the burying region. By proper choice of the difference between the effective refractive index of the fundamental transverse mode in the mesa (the mode in the direction perpendicular to the plane of the layers) and the refractive index of the burying region and the width of the mesa in the lateral direction, operation of the laser in the fundamental lateral mode can be obtained while operation in higher order lateral modes is suppressed. However, the useful range of output powers is limited by the onset of oscillation in higher order lateral modes and by the small emitting area of such devices.
It would be useful to have a buried heterostructure laser which operates only in the fundamental lateral mode over an increased range of output powers and which has a larger emitting area.